a terrifying scene in london at buckingham palace. the man with knives, the police who had to act fast. >>> the super bowl's superstar. which celebrity will win off the field? and will anyone be able to tackle betty white's moment. >> you're playing like betty white out there. >> that's not what your girlfriend says. >> baby! >>> from abc news world headquarters, this is abc world news with david muir. >>> good evening. thank you for being here on this sunday night. we begin this evening with that unfolding tragedy a former navy s.e.a.l., a hero to members of the military. this evening authorities say he was targeted and killed by a fellow veteran. he took out more enemies than anyone else did as a sniper. he often said he preferred to think about the lives he saved doing it. he returned to help american troops recover from the invisible wounds of war. tonight this young man, eddie routh, is accused of killing him. ryan owens leads us off tonight. >> reporter: former navy s.e.a.l. sniper chris kyle was such a good shot he once hit a t